As an English speaker, I'd be much more likely to say "ON average" than "IN average" in this instance.
I spelled pineapple as "pinaplpe" wtf
That's a lot of pineapple!
i am constantly wondering if duolingo aims on teaching us english or the other languages :D I use english almost everyday for ~5 or more years at full proficiency and still get caught mostly at english typos and mistakes :D
I translated it as "on the average"--a perfectly fine English locution, but it was marked wrong.